Page 295 - Handbook of Gold Exploration and Evaluation

Sedimentation and detrital gold 261

4.37 Desert placer gold model.

loads. Some of the gold is deposited in `gulch' placers the remainder of the
auriferous debris is carried on into an alluvial fan system in an open valley
setting at the base of the slope. The streams do most of their transporting and
sorting during flood times and deposit most of their spoil when the floods
recede. The duration of the active placer-forming processes is a critical factor in
the development of an economically viable deposit.

Deltaic placers
The name `delta' was given to the outpourings of the Nile by a Greek
philosopher Herodotus (484±424 BC) who recognised the similarity of the shape
to the fourth letter of the Greek alphabet. The delta classification is now based
upon formation rather than shape and the term is applied generally to all tracts of
alluvial ground between diverging branches of rivers where they empty into
large bodies of standing water. Deltas occur as freshwater deposits when the
streams discharge into lakes and as marine deposits when they flow into shallow
waters of the open sea.
Deltas formed under lacustrine conditions comprise simple arrangements of
topset, foreset and bottomset beds which increase in size and complexity with
differences in sediment supply and changes in the action of waves, tides and
currents. Changing rates of discharge and the depth of water at the river mouth
influences rate of growth. The Witwatersrand conglomerate deposits of South
Africa were laid down along the southwestern shoreline of an extensive inland
lake in the Witwatersrand Basin during the Archaean-Proterozoic transition 2.8±
2.2 billion years ago (see Chapter 2).

Bulolo lacustrine deposits
The Bulolo gold placer in Papua New Guinea is an example of Cainozoic gold
placers of deltaic origin. Prior to faulting in the Bulolo Valley, Papua New